1. Field of the Invention
This invention relates to porous silica microspheres which are useful as chromatographic material.
2. Background of the Art
This invention represents improvements in chromatographic material comprising porous silica microspheres. One improvement resides in an enriched concentration of silanol groups on the surface of crush-resistant microspheres. The higher level of surface silanol groups allows improved silanizations which produce microspheres having enhanced chromatographic properties.
U.S. Pat. No. 3,782,075, issued to Kirkland, discloses an improved packing material for chromatographic columns. The packing material comprises a plurality of uniform-sized porous microspheres having an average diameter of about 0.5 to about 20 .mu.m. The microspheres consist essentially of a plurality of uniform-sized colloidal particles, having a refractory metal oxide surface arranged in an interconnected three-dimensional lattice. The colloidal particles occupy less than 50% of the volume of the microspheres with the remaining volume being occupied by interconnected pores having a uniform pore size distribution.
U.S. Pat. No. 3,857,924, issued to Halasz et al., discloses a process for the production of spherical, porous silica particles. The process comprises treating an alkali polysilicate solution having a silica content of from about 5 to 7.5 percent by weight batchwise with a cation exchange material to remove cations, and thereafter batchwise with an anion exchange material to remove mineral acids. The treated solution is emulsified and coagulated in a water-immiscible organic medium thereby forming the silica particles. The silica particles are disclosed as having surfaces covered with a certain amount of silanol groups and are used as supports in chromatography, in catalytic processes as catalysts, as carriers for catalytically active materials, and so on.
U.S. Pat. No. 4,131,542, issued to Bergna et al., discloses a process for preparing a low-cost silica packing for chromatography. The process involves spray-drying an aqueous silica sol containing from 5 to 60 weight percent silica to form micrograins. These porous silica micrograins are acid-washed and sintered to effect a 5 to 20% loss in surface area.
U.S. Pat. No. 4,477,492, issued to Bergna et al., discloses a process for preparing superficially porous macroparticles for use in chromatography and as catalysts or catalyst supports. The process comprises spray-drying a specified well-mixed slurry of core macroparticles, colloidal inorganic microparticles and a liquid. The resulting product is dried and sintered to cause a 5%-30% decrease in surface area.
U.S. Pat. No. 4,010,242, issued to Iler et al., discloses oxide microspheres having a diameter in the 0.5 to 20 .mu.m range. The microspheres are produced by forming a mixture of urea or melamine and formaldehyde in an aqueous sol containing colloidal oxide particles. Copolymerization of the organic constituents produces coacervation of the organic material into microparticles containing the organic material. The organic constituent can be burned out to form a powder of uniform-sized porous microparticles consisting of an interconnected array of inorganic colloidal particles separated by uniform-sized pores.
U.S. Pat. No. 4,105,426, issued to Iler et al., discloses a powder of discrete, macroporous microspheroids, each having an average diameter in the range of 2 to 50 .mu.m. Each microspheroid is composed of a plurality of large colloidal particles joined and cemented together at their points of contact by 1 to 10% by weight of nonporous, amorphous silica. The microspheroids have a high degree of mechanical stability and a surface area between about 80 and 110% of that of the large colloidal particles. A process for the manufacture of the powder is also disclosed.
It is known that porous silica microspheres silanized with a uniform coating of organosilyl groups are efficient chromatographic material for separating various types of organic molecules from mixtures. In order to covalently attach these silyl groups, there must be silanol (Si--OH) groups on the silica surface. Another important characteristic for a chromatographic material is crush resistance so that beds of material are stable for use at high pressure. It is known to strengthen porous silica microspheres by heating at about 900.degree. C. After heat strengthening, there are very few silanol groups left on the surface of the silica. Instead, the surface is largely dehydroxylated to siloxane groups (SiOSi) which generally do not react with silanizing agents. A chromatographic material comprising crush-resistant silica microspheres of uniform pore size distribution having a high surface concentration of silanol groups is desirable.